Abstract
Cyclic α-maltosyl-(1→6)-maltose (CMM) is a cyclic glucotetrasaccharide with alternating α-1,4 and α-1,6 linkages. Here, we report functional and structural analyses on CMM-binding protein (CMMBP), which is a substrate-binding protein (SBP) of an ABC importer system of the bacteria Arthrobacter globiformis. Isothermal titration calorimetry analysis revealed that CMMBP specifically bound to CMM with a Kd value of 9.6 nM. The crystal structure of CMMBP was determined at a resolution of 1.47 Å, and a panose molecule was bound in a cleft between two domains. To delineate its structural features, the crystal structure of CMMBP was compared with other SBPs specific for carbohydrates, such as cyclic α-nigerosyl-(1→6)-nigerose and cyclodextrins. These results indicate that A. globiformis has a unique metabolic pathway specialized for CMM.
Highlights
ATP-binding cassettes (ABC) transporters constitute a membrane protein family that mediates diverse ATP-driven transport [1]
In the presence of dextran (>90% α-1,6 linkages) and pullulan, CMM-binding protein (CMMBP) exhibited reduced mobility (Fig 2C and 2D), suggesting that CMMBP interacts with those polysaccharides
We performed a functional and structural analysis of CMMBP (CmmC), a putative substratebinding protein (SBP) involved in a bacterial ABC importer system
Summary
ABC transporters constitute a membrane protein family that mediates diverse ATP-driven transport [1]. They basically consist of a pair of cytoplasmic domains, called ATP-binding cassettes (ABC) or nucleotide-binding domains (NBD), and transmembrane domains (TMDs) in the cell membrane [2]. ABC importers facilitate efficient intakes of substrates across the cell membrane, and bacterial importers usually work in conjunction with respective substratebinding protein (SBP) that binds the substrate [3]. As a general transport mechanism of bacterial ABC importers, SBP captures a substrate and delivers it to TMDs, and ATP hydrolysis by NBDs drives conformational changes of TMDs that translocate the substrate into the cytoplasm.
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